Oil and gas wells are usually lined with a steel pipe forming a casing which may hang freely or may be cemented in position by pumping a cement slurry between the outer surface of the pipe and a bore hole in which the pipe is located. In certain circumstances it may be necessary or desirable to cut the casing by milling at a distance from the well surface and to mill away a substantial length of pipe. Milling may be required, for example, to remove cemented casing so that a well can be redrilled or to remove a section of the casing to improve or permit oil and gas production at a desired elevation in a well.
It is known to use milling tools having either fixed or hydraulically radially expandable cutting blades. Known fixed diameter mills usually have a plurality of fin like cuttingblades radially projecting outwardly from a tubular body and the fins may be welded, brazed or bolted to the mill body. Known hydraulically activated mills have a plurality of cutting blades, often called `knives`, that are circumferentially disposed about a mill body and are pivotally attached to the body at an upper end of the blades so that the lower end of the blades may be swung radially outwardly when the mill has reached a desired location within the casing at which milling is to commence. A usual manner of radially opening the blades is by a reciprocally operable piston located within a longitudinal bore within the mill body, the piston being operable by circulation of drilling mud to force the piston downwardly. The piston is arranged to contact a cam surface on the blades to pivotally rotate the lower, in operation, end of the blades radially outwardly and thereby wedge the blades into contact with the casing to be milled. The mill body is connected to a drill string and the string is rotated to effect milling, the blades being maintained in the open position by the hydraulic action of the drilling mud on the piston.
Another type of milling tool is that known as a washover shoe which is a fixed diameter mill having a tubular body with cutting elements disposed around the lower periphery of the body. A washover shoe is used to mill away tool obstructions such as stabiliser ribs, reammer cutters, expanded packers and bit bodies which may be retaining a drill string downhole. By using a number of wash pipes, the rotatable washover shoe is passed over the drill string and lowered to the position of the obstruction so that, in effect, the washover shoe cuts an annulus.
It has been conventional to use fragments of crushed tungsten carbide secured in a layer of brazing alloy on the part of the blade which, during rotation thereof, is the leading face i.e. that portion of the blade which is forwardly facing during rotation. The brazing process leaves particles of carbide in a more or less randomly distributed fashion and orientation in the brazing alloy. Such random orientation of the tungsten carbide fragments and hence the cutting edges of the fragments significantly limits the milling efficiency of the tool and creates mainly undesirably long cuttings which may cause the mill or the drill string to become stuck. The total amount of tungsten carbide fragments available for milling is limited by the need to have a supporting matrix of brazing alloy.
So as to overcome the problems associated by the random orientation and distribution of the tungsten carbide fragments, mills have been constructed with geometrically shaped cutting elements and such a mill is disclosed in U.S. Pat. No. 4,710,074 assigned to Smith International Inc. In such a mill the blades have a leading cutting edge on which is secured tungsten carbide cutting elements across the leading face of the blade in a radial row and there being a plurality of rows extending in the longitudinal direction of the axis of the milling tool. The total number of cutting elements that can be used on such a milling blade is limited to the surface area of the radial cutting blade. The cutting elements have a rectangular cross section in the longitudinal, axial, direction with the depth of each element in the rotational direction being significantly less than the height of each element in the longitudinal axial direction. The leading face of the elements is either parallel to the longitudinal axis of the mill body or tilted such that the upper edge in use of each element is inclined forwardly of the lower edge of said element to provide what is known as negative axial rake. The provision of such negative axial rake is believed to provide more efficient cutting and to provide shorter cuttings that can be circulated out of the well more conveniently by drilling mud. The tungsten carbide elements are usually secured to the front, i.e. leading, face of the mill by brazing but the complete front face of the cutting elements is unprotected against axial, torsional and radial shocks that are frequently encountered during casing milling operations. The tungsten carbide elements therefore tend to crack and break off the cutting blades which, it will be realised by those skilled in the art, limits the distance that can be milled with a single mill and which can be milled in a continuous operation.